Convection homogenizes magma intrusions
DOI: 10.1063/1.3595138
Convection homogenizes magma intrusions. Plutons are mountain-sized formations of igneous rock that poke through Earth’s surface. Their origin as solidified eruptions of magma is straightforward to explain. What’s puzzling is why plutons are so homogeneous on large scales, despite their immense size and despite being inhomogeneous in their mineral composition on small scales. Alain Burgisser of the Institute of Earth Sciences in Orléans, France, and George Bergantz of the University of Washington in Seattle have proposed an answer. In their model, a mass of viscous, semisolid magma—”mush” is the technical term—lies beneath the surface, hemmed in by walls of more solid rock. Pluton formation begins when the slow churning of the mantle below happens to bring a body of hot magma into contact with the bottom surface of the cooler mush. Over the ensuing decades, the heat rises slowly via conduction, making the mush less viscous and—crucially—less dense. In a process that Burgisser and Bergantz call unzipping, the gradually growing layer of hot, light mush abruptly undergoes a Rayleigh–Taylor instability, which sends convective plumes of hot mush upward through the nascent pluton. Within a few months, the first plumes reach the top of the nascent pluton, cool, then sink. Only a few successive cycles of overturning suffice to homogenize the mush on plutonic scales. Burgisser and Bergantz’s model can plausibly account for the speed with which three real plutons formed, including the one left by the 1991 eruption of Mount Pinatubo in the Philippines. (
